The present disclosure is generally related to the field of color image/text printing or display systems and to methods and systems for calibrating color output devices, such as color displays, printers and printing devices thereof. Color has become essential as a component of communication and facilitates the sharing of knowledge and ideas, and there are continuous efforts to improve the accuracy and total image quality of digital color output devices.
Color images are commonly represented as one or more separations, each separation comprising a set of color density signals for a single primary or secondary color. Color density signals are commonly represented as digital gray or contone pixels, varying in magnitude from a minimum to a maximum, with a number of gradients between corresponding to the bit density of the system. Thus, a common 8-bit system provides 256 shades of each primary color. A color can therefore be considered the combination of magnitudes of each pixel, which when viewed together, present the combination color. Usually, printer signals include three subtractive primary color signals (i.e., Cyan, Magenta and Yellow) and a Black signal, which together can be considered the printer colorant signals. Each color signal forms a separation and when combined together with the other separations, forms the color image.
In order to facilitate the exchange and reuse of documents it is desirable to specify document properties in a printer-independent fashion, where possible. Colors are therefore preferably specified in a printer-independent color space based on the characteristics of human vision. Since the native control spaces of output devices (e.g., a printer's CMYK values) are not printer-independent color spaces, in order to print or display a given color it is necessary to determine the device control values corresponding to specified printer-independent color values. This is normally accomplished by a three-step procedure.
First, a set of color patches with pre-determined device control values is output on the device and the color of each patch is measured in printer-independent color coordinates. Second, using the device control values and the corresponding measured printer-independent color values; a “forward device-response function” or forward transform is estimated. The “forward device-response function” represents the mapping from device control values to the printer-independent-color values produced by the device in response to the control values. Third, the “forward device-response function” or forward transform can be “inverted” to obtain a “device-correction-function” or inverse transform. This transform or “device-correction-function” maps each printer-independent color to the device control values that produce the specified printer-independent color value on the output device. The “device-correction-function” is typically pre-computed and stored. In order to produce a given color on the output device, the corresponding printer-independent color values are mapped through the “device correction-function” to obtain control values. When the device is driven with these control values, the desired color is produced.
It is common practice to separate the “device correction-function” into two parts: a “calibration” function that immediately precedes the device and a “characterization” function, which addresses the device “through” the calibration function. This separation is illustrated in FIG. 5 for the case of a CMYK printer. FIG. 5 illustrates a conventional system 400, which partitions “device-correction function” into characterization and calibration. For example, system 400 includes a printer-independent-color which can be provided as input 410 to a characterization routine 402 whose output is fed to a calibration unit 404, whose output in turn is fed to an output device 406 via output line 414. Additionally, lines 408 and 412 indicate alternate CMYK paths used for fast reprint and fast emulation respectively, that are fed to calibration unit 404.
Another example of a calibration system includes U.S. Pat. No. 5,305,119 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Apr. 19, 1994 and is assigned to Xerox Corporation. U.S. Pat. No. 5,305,119 is generally directed toward a method of calibrating a response of a printer to an image described in terms of colorimetric values. A further example of a calibration method and system is described in U.S. Pat. No. 5,528,386 to Rolleston et al, “Color Printer Calibration Architecture,” which issued on Jun. 18, 1996 and is also assigned to Xerox Corporation. U.S. Pat. No. 5,528,386 generally describes a conventional one-dimensional architecture. Both U.S. Pat. Nos. 5,305,119 and 5,528,386 are incorporated herein by reference, and are referenced for general edification and illustrative purposes only.
The purpose of the calibration transformation is to facilitate a trade-off. Unlike the full device-correction function, the calibration transformation provides control of the output device only in a limited fashion. However, in comparison to the full device-correction function the calibration transformation also offers significant advantages in that it requires substantially reduced measurement effort and also a substantially lower computational effort. The lower computational effort requirement allows it to be incorporated in high-speed real-time printing image-processing chains for which the full device-correction function may be too computation and/or memory intensive. For color output devices, particularly those utilized in the printing arts, calibration is typically performed for the Black channel (K) independently and for the Cyan (C), Magenta (M), and Yellow (Y) channels either independently or together.
As an illustrative example, consider the case of a 3-channel (CMY) printer is considered. The goal of calibration is to determine a calibration transform from CMY to C′M′Y′ that maintains a desired printer response in selected regions of color space. Additionally, the calibration transform is required to be computationally efficient with a small memory requirement so that it can be incorporated in high-speed real-time printing paths. Traditionally, the calibration transform has been applied in the form of one-dimensional correction to the individual channels for the device. For CMY, the calibration is applied as tone-response corrections (TRCs) for each of the C, M, and Y channels. FIG. 6 illustrates a traditional three-color one-dimensional calibration transformation system 500. As indicated in FIG. 6, arrows 502, 504 and 506 respectively represent C, M, and Y inputs to transformations 514, 516, and 518. Dashed line 512 generally indicates the boundaries of a calibration transformation, which is composed of individual transformations 514, 516, and 518. Outputs are indicated by arrows 508, 510 and 511, which respectively represent C′, M′ and Y′. It can thus be appreciated that the following equation applies to system 500 of FIG. 6:C′=f1(C), M′=f2(M), Y′=f3(Y)
Calibration and/or characterization of color printers is often subject to different forms of noise. In particular one type of noise occurs in the form of discrepancies between the printer-dependent CMY or CMYK output values produced by the inverse transform for given printer-independent input colors, and the true input printer-dependent CMY or CMYK values that originally produced those printer-independent colors. Such noise may be attributable, at least partially, to spatial non-uniformity of the printer, where providing identical CMY or CMYK values to two patches at different spatial x and y locations on the page can result in the printing of two colors that are perceived and measured as being distinctly different. Spatial non-uniformity may generally occur in the form of banding, streaks and mottle. Moreover, the spatial non-uniformity may not repeat the same noise pattern from one page to the next, and consequently may be difficult to correct prior to printing. Prior attempts to combat printer spatial non-uniformity include randomizing the locations of printed test patches and averaging of large numbers of measurements to reduce the noise. However, this approach requires dedication of measurement equipment for long periods of time and is thus impractical in many applications. Consequently, there remains a need for improved color printer calibration and characterization techniques by which the effects of spatial non-uniformity can be mitigated without significantly increasing calibration or characterization time or expense.